Indolicidin is an antimicrobial peptide-amide isolated from the cytoplasmic granules of bovine neutrophils. The antimicrobial mechanism of action of this peptide probably involves the disruption of cell membranes. Its remarkably high tryptophan content (five of thirteen residues), indicative of a highly membrane-active peptide is consistent with such a mechanism. We intend to use fluorescence approaches to study interaction of indolicidin and its analogs with lipid membranes. Specifically we will be studying peptide's possible aggregation and translocation across the bilayer by means of time-resolved and steady-state fluorescence quenching and energy transfer. However, our preliminary data indicate existence of a substantial energy homotransfer between the tryptophanes which impairs our ability to do such studies. We have synthesized an all phenylalanine analog of indolicidin (all Trp's replaced with Phe's) which exhibits a similar antimicrobial activity. We now plan to systematically substitute back Trp residues one at a time and use those analogs for structural studies.